The present invention relates to multi-slice helical computerized tomography and more particularly to an algorithm, method and apparatus for using the same which reduces the data acquisition time and data processing time required to generate an image.
In computerized tomography (CT) X-ray photon rays are directed through a patient toward a detector. Attenuated rays are detected by the detector, the amount of attenuation indicative of the make up (e.g. bone, flesh, air pocket, etc.) of the patient through which the rays traversed. The attenuation data is then processed and back-projected according to a reconstruction algorithm to generate an image of the patient's internal anatomy. Generally, the "back projection" is performed in software but, as the name implies, is akin to physically projecting rays from many different angles within an image plane through the image plane, the values of rays passing through the same image voxels being combined in some manner to have a combined effect on the voxel in the resulting image. Hereinafter the data corresponding to rays which are back projected will be referred to as back projection rays.
During data acquisition, if a patient moves, artifacts can occur in the resulting image which often render images useless or difficult to use for diagnostics purposes. For this and other reasons, as in other imaging techniques, the CT industry is constantly trying to identify ways to reduce the duration of acquisition periods without reducing the quality of the data acquired.
In addition, because huge amounts of data are acquired during an acquisition period and the processing methods for image reconstruction from the gathered data are relatively complex, a huge number of calculations are required to process data and reconstruct an image. Because of the huge number of required calculations, the time required to process collected data and reconstruct an image is appreciable. For this reason the CT industry is also constantly searching for new processing methods and algorithms which can speed up the reconstruction process.
Various CT system features and procedures have been developed to increase data acquisition speed and to speed up the reconstruction process. Some of the more popular features and procedures including fan beam acquisition, simultaneous multiple slice acquisition, helical scanning and half-scanning. In fan beam acquisition the source is collimated into a thin fan beam which is directed at a detector on a side opposite a patient. In this manner, a complete fan beam projection data set is instantaneously generated for a beam angle defined by a central ray of the source fan beam. The source and detector are rotated about an image plane to collect data from all (e.g., typically 360 degrees) beam angles. Thereafter the collected data is used to reconstruct an image in the image plane. Thus, fan beam acquisition reduces acquisition period duration.
With respect to half-scanning, assuming a patient remains still during a data acquisition period, conjugate data acquisitions (i.e., data acquired along the same path from opposite directions) should be identical. In addition, using a fan beam, at least one ray can be directed through an image plane from every possible beam angle without having to perform a complete rotation about the patient.
For example, referring to FIG. 3, an annular gantry opening 70 is illustrated with a patient slice 42 disposed (support table not illustrated) therein and with respect to a Cartesian coordinate system where the Z-axis is into the Figure and defines a transport axis. A source 10 is illustrated in first, second, third and fourth positions as 90, 90', 90" and 90'", respectively. When in the first position, source 10 generates a fan beam 40 which includes a central ray Rc and additional rays diverging therefrom along fan angles, the maximum fan angle being .GAMMA.. The beam angle B is defined as the angle formed by central ray Rc with respect to the vertical Y-axis.
When in the fourth position, source 10 generates a fan beam 40'" which also includes a central ray (not illustrated) and rays diverging therefrom to form the fan beam. By rotating the source from the first to the fourth position in a clockwise direction data is collected at least once from every possible beam angle through slice 42 (i.e., the image plane). As known in the industry, data corresponding to every beam angle corresponding to a single image plane can be collected after a (.pi.+2.GAMMA.)/2.pi. rotation about the patient. Because less than an entire rotation about the image plane is required to acquire the imaging data these acquisition methods and systems are generally referred to as half-scan methods and systems. Thus, half-scan acquisition has been employed to reduce acquisition period duration in conjunction with single row detectors.
In addition, because relatively less data has to be processed in the case of half-scan imaging methods and systems to generate an image, half-scan methods and systems also have the advantage of potentially reducing data processing and reconstruction times.
While fan beams and half-scans have several advantages, often, during a diagnostics exercise a system user typically will not know the precise location within a patient of an object, cavity, etc. of interest to be imaged. For this reason, it is advantageous for a system user to be able to generate several cross sectional images in rapid succession by selecting different image/reconstruction planes. In these cases rapid data processing is extremely important to minimize delays between image generation so that a user does not lose her train of thought between image views.
Single slice detectors, fan beams and half-scans can be used to generate data in several different parallel image planes which, after data acquisition, can be used by a processor to generate an image anywhere between the image planes through interpolation/extrapolation procedures known in the art. For example, assume that during two data acquisition periods first and second data sets were acquired which correspond to first and second parallel acquisition planes, respectively, the planes separated by 0.25 inches. If a user selects an image plane for reconstructing an image which resides between the first and second acquisition planes, interpolation between data in the first and second sets can be used to estimate values of data corresponding to the selected image plane. For instance, assume that, among other rays, during the acquisition periods a first ray and a second ray were used to generate data in the first and second sets, respectively, and that the first and second rays were parallel (i.e. had the same beam and fan angles). In this case, by interpolating between the data acquired from the first and second rays generates an estimated value corresponding to a hypothetical back projection ray which is parallel to the first and second rays and which is within the image plane. By performing such interpolation to generate back projection rays for every beam and fan angle through the image plane a complete data set corresponding to the image plane is generated.
While such systems work, unfortunately, the acquisition time required to generate data corresponding to many image planes is excessive and inevitable patient movement often causes image artifacts.
One way to speed up data acquisition corresponding to several image planes is by employing a multi-row detector with a fan beam. In multi-row detector systems, a relatively thick fan beam is collimated and directed at a multi-row detector with a patient there between, each detector row in effect gathering data for a separate "slice" of the thick fan beam along the Z or translation axis perpendicular to a fan beam width. Despite each detector row having a thickness, in these systems it is assumed that the detected signals in each row correspond to a plane centered within the row as projected onto the isocenter Z. Hereinafter the central plane through a row will be referred to as a row center.
After data acquisition an interface enables a system user to select an image plane from within the area corresponding to the collected data. The selected image plane is between the row centers of at least two adjacent detector rows. After image plane selection, a processor interpolates between data corresponding to adjacent rows to generate back projection rays corresponding to the selected image plane. When another image corresponding to a different image plane is desired, after selecting the plane, the processor again identifies an acquired data subset for interpolation, additional processing and back projection. Thus, multi-row detector systems further reduce data acquisition period duration where several image planes may be selected for reconstruction.
One limitation with multi-row detectors is that, during a single acquisition period, data can only be collected which corresponds to the detector thickness. To collect additional data corresponding to a greater patient volume, after one acquisition period corresponding to a first volume, the patient has to be moved along a translation axis until a second volume which is adjacent the first volume is between the source and detector. Thereafter a second acquisition process has to be performed. Similarly, to collect additional data corresponding to a third volume the patient has to be transported to another relative location with respect to the source and detector. Required translation without acquisition necessarily prolong the acquisition period and the additional acquisition time and aligning processes inevitably result in relative discomfort, additional patient movements and undesirable image artifacts.
Helical scanning systems have been developed so that data can be collected during a single acquisition period without halting patient translation during the acquisition period. In a helical scanning system, the source and detector array are mounted on opposing surfaces of an annular gantry and are rotated there around as a patient is transported at constant speed through the gantry. The X-ray beam sweeps a helical path through the patient, hence the nomenclature "helical scanning system". Data acquisition can be sped up by increasing operating pitch (i.e., table translation speed relative to gantry rotation rate). After data is acquired the data is processed to generate back projection ray estimates and account for data nuances which are caused by the helical acquisition.
Various combinations of the fan-beam, multi-slice, half-scan and helical scanning features have been combined to realize synergies and have been somewhat successful. For example, one system combines a multi-row fan beam detector and a fan beam source with a helical scanning procedure to rapidly acquire imaging data using a high pitch/high speed mode. For example, an exemplary system including a four row detector may support a 6:1 pitch wherein the detector crosses a reconstruction plane in 0.67 rotations. It is envisioned that an eight slice scanner will be able to support an 11:1 pitch so that the detector crosses a reconstruction plane in 0.73 rotations.
Referring again to FIG. 3, in these high speed helical scanning systems, during acquisition data is acquired with source 10 at position 90, the source and detector are rotated (while data is collected) about gantry opening 70 as the patient 42 is transported there through. A processor collects data during transport and rotation from many different beam and fan angles. After source 10 rotates through a complete rotation and reaches position 90 again, additional data is gathered at that position. Because the patient 42 is transported along the Z axis during acquisition, while source 10 is at the same location 90 relative to opening 70 at the beginning and at the end of the rotation, the source and data collected are at a different Z location relative to patient 42. Hereinafter data collected for the same beam and fan angles but at different Z locations will be referred to as consecutively collected data.
Current interpolation techniques interpolate between consecutively collected data (i.e., data from source 10 at the same beam angle (e.g., position 90 in FIG. 3) and fan angle but at different Z (i.e., translation axis) locations. In other words, current interpolation techniques require data from more than a single source rotation to generate an image. In addition, because data from more than one rotation is required to interpolate, the data collection is relatively large and processing and reconstruction period durations are excessive. Moreover, because interpolation is between consecutively collected data, the resulting image has a "thickness" characteristic which corresponds to a relatively thick patient volume which is unsuitable or at least not optimal for many diagnostic purposes.